Horton

Re: Improve CRC32C performance on SSE4.2

Soumyadeep Chakraborty <soumyadeep2007@gmail.com>

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From: Soumyadeep Chakraborty <soumyadeep2007@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andy Fan <zhihuifan1213@163.com>, "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>, Jesper Pedersen <jesperpedersen.db@gmail.com>, Tomas Vondra <tomas@vondra.me>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, "Shankaran, Akash" <akash.shankaran@intel.com>
Date: 2025-07-13T19:28:11Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Include _mm512_zextsi128_si512() in AVX-512 configure probes.

  2. Properly fix AVX-512 CRC calculation bug

  3. Workaround code generation bug in clang

  4. Compute CRC32C using AVX-512 instructions where available

  5. Inline CRC computation for small fixed-length input on x86

  6. Be more paranoid in configure's checks for CRC and POPCNT intrinsics.

On Tue, Jun 17, 2025 at 1:55 AM John Naylor <johncnaylorls@gmail.com> wrote:

I took the minimal repro from [1] and took a look at the code generated
between clang 17 -O0 [2] and clang 17 -O3 [3]. I saw that -O3 (and
actually -O1 and -O2) generated the following code for:

castval = _mm512_castsi128_si512(_mm_cvtsi32_si128(crc0));
x0 = _mm512_xor_si512(castval, x0);

vinserti128  ymm0, ymm0, xmmword ptr [rip + .LCPI1_0], 0
vpxorq  zmm0, zmm0, zmmword ptr [rdi]

Reading vpxorq's pseudocode [4], it seems that it zeroes out the leading
bits:

DEST[MAXVL-1:VL] := 0

Same thing for clang 17 -O0, if we are using _mm512_zextsi128_si512
instead [5] -  vpxor and vbroadcast128 are used which seem to also
zero out leading bits.

So, -O1..-O3 were indeed emitting instructions that zero-extend and, thus
avoiding the undefined behavior.

[1]
https://www.postgresql.org/message-id/PH8PR11MB8286A89AF2B104044187E54DFB70A%40PH8PR11MB8286.namprd11.prod.outlook.com
[2] https://godbolt.org/z/ahx9PePYr
[3] https://godbolt.org/z/W4WPzjnbb
[4] https://www.felixcloutier.com/x86/pxor#vpxorq--evex-encoded-versions-
[5] https://godbolt.org/z/46brvrnnv

Regards,
Deep (VMware)